CN115398309A

CN115398309A - Optical limiter and method for limiting radiant flux

Info

Publication number: CN115398309A
Application number: CN202180023671.8A
Authority: CN
Inventors: M·门切蒂
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 2020-03-30
Filing date: 2021-03-10
Publication date: 2022-11-25
Also published as: EP4094114A1; US20230140182A1; WO2021197780A1; US11774785B2

Abstract

One aspect relates to an optical limiter (200) for limiting a radiant flux of a source beam, the optical limiter comprising: a light control port (221) illuminated by a light control beam originating from the source beam; a light input port (221) to be illuminated by a light transmission beam originating from the source beam; a light output port (225) illuminated by the transmitted beam; and thermally driven phototubes (211, 212); wherein the optical windmill is arranged relative to the input port, the control port and the output port such that: the control port is windmilled to rotate only if the control beam has a radiant flux equal to or exceeding a predetermined radiant flux threshold; and the rotation of the photo-windmill causes the area of the output port illuminated by the transmitted beam to change.

Description

Optical limiter and method for limiting radiant flux

Technical Field

The present disclosure relates to limiting the radiant flux of a beam; i.e. the radiation energy transmitted by the beam per unit time-its power-is limited.

More particularly, aspects relate to an optical limiter and a method for limiting the radiant flux of a source beam.

Background

Optical networks are used to transmit data encoded as optical signals over fiber optic cables. To ensure that the optical signal is successfully transmitted from the source to the destination without damaging any network components, it may be desirable to limit the radiant flux of the optical beam carrying the optical signal. This can be achieved using an optical limiter.

An optical limiter is a device for limiting the radiant flux of a beam of light to not exceed a predetermined maximum value. Fig. 1A1 shows an ideal plot of input radiant flux versus output radiant flux for a flat maximum optical limiter, where the output rises in proportion to the input until a maximum output value M is reached, at which point the output remains at that level M regardless of how much the input rises.

The optical limiter may be formed, for example, using a material having a negative thermal index coefficient, wherein the refractive index of the material is lowered by absorbing heat generated by the light beam, causing the light rays to fan out in a defocused pattern so that the collimating lens receives only some of these light rays. Other types of optical limiters use a stable optical amplifier whose output is kept constant by a feedback loop or exhibits saturation at its input.

An optical fuse is a particular type of optical limiter used to interrupt the passage of a beam when the radiant flux of the beam exceeds a predetermined maximum value. A plot of input radiant flux versus output radiant flux for an idealized photo-fuse is shown in fig. 1A2, where the output rises in proportion to the input until a maximum output value M is reached, at which point the output drops to zero and remains zero for all higher input values.

The optical fuses may be constructed, for example, using light absorbing materials that are either destroyed by heat generated when an intense beam of light is incident upon them, or whose transmissivity is altered by the heat (e.g., such that they become opaque). Therefore, the optical fuses are generally disposable; they must be replaced to re-establish the optical connection along the path in which they reside.

What is needed is an alternative optical limiter that is reusable and does not require the use of special materials.

Disclosure of Invention

According to a first aspect, there is provided an optical limiter for limiting radiant flux of a source beam, the limiter comprising:

a light control port illuminated by a light control beam originating from the source beam;

a light input port illuminated by a light transmission beam originating from the source beam;

an optical output port illuminated by the transmission beam; and

a thermally driven photo-windmill;

wherein the optical windmill is arranged relative to the input port, the control port and the output port such that:

the control port is powered to rotate by the control beam only if the control beam has a radiant flux equal to or exceeding a predetermined radiant flux threshold; and

rotation of the photo-mill causes the area of the output port illuminated by the transmitted beam to change.

The optical windmill includes:

a shaft; and

at least one blade:

is arranged to rotate about the axis of the shaft in an environment containing a fluid, and

having a first side and a second side thermally insulated from each other such that the windmill is driven by irradiation of the first side by the control beam such that the first side absorbs more light energy than the second side, establishing a temperature gradient from the second side to the first side such that the blade rotates about the axis of the shaft while the first side is being towed.

The predetermined radiant flux threshold depends on the inertia of the photo-wind mill.

The limiter may be configured to allow the optical windmill to rotate a sufficient degree so that the area of the output port illuminated by the transmission beam is zero, so that the limiter acts as a reusable optical fuse.

The source beam may be laser-derived.

The side of the blade of the photo-windmill that is arranged to be illuminated by the control beam may have a higher light absorption than the opposite side of the blade.

Alternatively or additionally, the side of the blade of the photo-windmill that is arranged to be illuminated by the control beam and the opposite side of the blade may be shaped such that, within the allowed rotational range of the photo-windmill, the side of the blade illuminated by the control beam receives a greater amount of radiant energy from the control beam than the opposite side.

The limiter may further comprise optical baffle means arranged to prevent a portion of the transmitted beam from impinging the output port, the size of the portion being dependent on the angle of rotation of the windmill.

The separator device may include:

one or more light guides; and/or

One or more beam blockers.

The input port and the output port may be coaxial with each other;

the baffle arrangement may comprise one or more beam blockers arranged to rotate with the photo-windmill; and is

The one or more beam blockers may be arranged to define an aperture through which the transmitted beam must pass to reach the output port.

When the angle is zero, the aperture may be coaxial with the input port and the output port.

The input port may be the control port, the source beam itself acting as both the transmission beam and the control beam.

The limiter may further comprise a beam splitter arranged to split the source beam into the transmission beam and the control beam.

The limiter may further comprise a biasing element;

wherein the phototool is coupled to the biasing element such that the phototool is biased toward a first rotational position in which an area of the output port illuminated by the transmission beam is maximized relative to any other rotational position of the phototool.

The biasing element may be configured to be adjustable such that the biasing force exerted by the biasing element on the windmill may be modified.

The biasing element may be configured to:

preventing the light windmill from rotating away from the first rotational position when the radiation flux of the source beam is below a predetermined threshold; and

when the radiant flux of the source beam equals or exceeds a predetermined threshold, the photo-windmill is allowed to rotate from the first rotational position by a sufficiently large angle such that the area of the output port illuminated by the transmission beam is zero.

The limiter may further comprise one or more stops, each stop being arranged to prevent the windmill from rotating past a particular rotational position.

The amplitude limiter may further comprise a housing enclosing the windmill, the housing comprising an aperture configured to partially evacuate fluid around the windmill.

According to a second aspect, there is provided a method of limiting radiant flux of a source beam, the method comprising:

illuminating the input port of the optical limiter according to any preceding claim with an optical transmission beam originating from the source beam; and

illuminating the control port of the optical limiter of any preceding claim with an optical control beam originating from the source beam.

Drawings

Aspects of the present disclosure will now be described, by way of example, with reference to the accompanying drawings. In the drawings:

FIG. 1A1 is an idealized graph of input radiant flux versus output radiant flux for a planar maximum optical limiter;

FIG. 1A2 is a graph of input radiation flux versus output radiation flux for an idealized fuse;

FIG. 1B1 shows a croakes radiometer light windmill;

FIG. 1B2 shows the forces on each blade of the croakes radiometer of FIG. 1B 1;

FIG. 1C shows another design of a photo-wind mill;

FIG. 2A illustrates an example optical limiter in a first position;

FIG. 2B shows the limiter of FIG. 2A in a second position;

FIG. 3A shows an example optical fuse in an "ON" position;

FIG. 3B shows the fuse of FIG. 3A in an "open" position;

FIG. 3C illustrates some of the internal components of the fuse of FIG. 3A;

FIG. 3D illustrates some of the external components of the fuse of FIG. 3A;

FIG. 4A illustrates another example optical limiter in a first position;

FIG. 4B shows the limiter of FIG. 4A in a second position;

FIG. 5A shows another example optical fuse in an "ON" position;

FIG. 5B shows the fuse of FIG. 5A in an "open" position;

FIG. 6A illustrates another example optical limiter in a first position;

FIG. 6B shows the limiter of FIG. 6A in a second position;

FIG. 7A shows another example optical fuse in an "on" position;

FIG. 7B shows the fuse of FIG. 7A in an "open" position;

FIG. 8A illustrates another example optical limiter in a first position;

FIG. 8B shows the limiter of FIG. 8A in a second position;

FIG. 9A shows another example optical fuse in an "on" position; and

FIG. 9B shows the fuse of FIG. 9A in an "open" position;

Detailed Description

The following description is presented to enable any person skilled in the art to make and use the system, and is provided in the context of a particular application. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art.

The terms "top," "bottom," "side," "front," "back," "forward," "rearward," "clockwise," "counterclockwise," and other terms describing the orientation of features are not intended to be limiting, and are included purely to describe the relative position of features in the context of the figures when used. In use or during storage, the features may be arranged in other orientations.

It is proposed to use the optical windmill effect to route an optical signal between the input and output of an optical limiter such that the radiant flux of the output beam carrying the signal is limited to not exceed a predetermined maximum value.

A thermally driven photo-windmill comprising at least one blade/vane, the two sides of which are thermally insulated from each other so that when one side is heated by light or other electromagnetic radiation, that side remains hotter than the other side. The blades are located in a fluid (typically low pressure air) so that convection currents established in the fluid due to temperature asymmetry between the two sides of each blade are sufficient to turn the windmill.

To start up a stationary windmill, the effect of rotating the windmill (and thus the radiant flux of the light source causing the effect) must be sufficient to overcome the inertia of the windmill. In order to keep the phototool rotating, the phototool effect (and hence the radiant flux of the light source causing the effect) must be sufficient to overcome the frictional forces acting on the phototool during rotation with respect to its mounting and the surrounding fluid.

Optionally, both sides of each blade may have different electromagnetic absorption characteristics in order to increase the rate at which a temperature difference is established and/or to allow a temperature difference to be established if both sides of the blade are exposed to radiation.

Fig. 1B1 shows a Crookes radiometer 1100, which is a classical demonstration of the optical windmill effect. The Crookes radiometer 1100 includes four blades 1110 arranged to rotate within a partially evacuated canister 1130 about an axis 1120. Each vane 1110 is white on one side and black on the other, and the vanes are arranged so that the black side and the white side alternate around the radiometer. (the black side is cross-hatched.) when light is irradiated onto the radiometer 1100 from the direction indicated by arrow L1, the blade is rotated about the shaft 1120 in the direction indicated by arrow R1 (i.e., the black side is dragged).

Fig. 1B2 shows the forces on each blade 1110 of the croakes radiometer 1100 of fig. 1B 1. In FIG. 1B2, the vanes 1110 are shown side-by-side with the white side 1111 on the left and the black side 1112 on the right. The white side 1111 and the black side 1112 are thermally insulated from each other. The black side 1112 absorbs more light energy than the white side 1111. Thus, a temperature gradient is established from the white side 1111 to the black side 1112 (left to right in fig. 1B 2). This establishes convection in the rarefied air surrounding the blades, resulting in unbalanced forces on the blades. The "thermal creep" force TC acts in the opposite direction to the temperature gradient. An additional "einstein effect" force E acts on the edge of the surface perpendicular to the temperature gradient, also in the opposite direction to the temperature gradient. Thus, forces TC and E cause the blade 1110 to rotate about the axis 1120 while the black side 1112 is dragged.

The Hettner radiometer is similar to the Crookes radiometer, but has horizontal (as opposed to vertical) blades, with the exposed face of each blade coated half-black and half-white, with the black and white sides alternating around the radiometer. There is no einstein effect in the Hettner radiometer, but due to thermal creep forces it is still rotating and the black side of the blade is dragged.

Fig. 1C is a plan view showing another design of a windmill 1200 in which the light absorption difference between the two sides of each blade 1210 is due to their geometry rather than color. The blades 1210 are shaped so that they each have a concave side 1211 and a convex side 1212, the convex and concave alternating around the radiometer. Shading in fig. 1C shows where the shading falls when light is irradiated onto the photo-windmill 1200 from the direction indicated by the arrow L2. It can be seen that the convex side 1212 of the blade 1210 receives more light energy throughout the rotation than does the concave side 1211. This establishes a temperature gradient from the concave side 1211 to the convex side 1212 of each blade 1210, such that the thermal creep forces and einstein effect cause the blade 1210 to rotate about the axis in the direction indicated by arrow R2, i.e., the convex side 1212 is dragged.

The exemplary windmill designs described above all include blades where the light absorption characteristics of the two sides of the blade are asymmetric. However, even without this asymmetry, the motion will be caused by thermal creep forces and (depending on the design geometry) the einstein effect, as long as a temperature gradient can be established between the two sides of the blade so that it is thermally driven.

Thermally driven phototools typically operate in a low pressure gas or gas mixture (e.g., air), but may generally operate in any fluid capable of carrying convection currents.

Although the exemplary windmill designs described above all include four blades, any number of blades will be subject to the windmill effect.

It will also be appreciated that the phototool may be driven by other forms of electromagnetic radiation than visible light, for example infrared or ultraviolet radiation may also be used.

Several example designs of an optical limiter for limiting the radiant flux of a source beam will now be described. Each example limiter includes an optical input port, an optical output port, and an optical control port. The control port is arranged to be illuminated by an optical control beam originating from the source beam. The input port is arranged to be illuminated by a light transmission beam also originating from the source beam. The output port is arranged to be illuminated by the transmission beam. Further, each exemplary limiter comprises a thermally driven windmill arranged such that illumination of the control port by the control beam drives the windmill in rotation only when the control beam has a radiant flux equal to or exceeding a predetermined radiant flux threshold. The rotation of the photo-windmill in turn causes the area of the output port illuminated by the transmitted beam to change. In this way, the radiant flux of the output beam emitted through the output port can be limited.

Fig. 2A and 2B show schematic plan views of an example optical limiter 200, the optical limiter 200 including an optical windmill in a first position and a second position, respectively. To avoid cluttering the two figures, reference numerals not relevant to the detailed description of each figure are omitted in the figure, although all components involved are present in the limiter 200 as shown in each of the two figures.

The photo-windmill includes a rigid assembly configured to rotate about a shaft 214, the rigid assembly being centered on the shaft 214. The rigid assembly comprises a first blade 211 rigidly connected to a second blade 212, the second blade 212 acting as a counterweight for the first blade 211. The phototool is located in a sealed housing 240 that maintains the air around the phototool at a low pressure but is not completely evacuated.

A relatively low power optical beam (e.g., a laser beam) is indicated in fig. 2A by relatively wide transverse cross-hatching. (both uses of "opposite" in the previous sentence are indicative of a comparison with FIG. 2B, which will be described below.) the light beam enters the housing 240 through the input port 221 to be incident on the first blade 211. The surface of the first blade 211 on which the light beam is incident is partially reflective, so a portion of the light beam is reflected off the first blade 211 to be incident on the output port 225. The various components of the limiter 200 are arranged so that when the windmill is in the position shown in figure 2A, all of the reflected portions of the beam are incident on the output port 225.

The surface of the first blade 211 on which the beam is incident is configured to absorb some of the electromagnetic radiation (the unreflected portion) carried by the beam. This surface is thermally insulated from the surface on the opposite side of the first blade 211, so that a temperature gradient occurs from the shadow side to the illuminated side. Thus, the photo-windmill effect tends to rotate the photo-windmill clockwise such that the illuminated first blade 211 recedes from the light beam incident thereon. However, the light beam shown in fig. 2A has a low enough power that the phototool effect is not sufficient to overcome the inertia of the phototool. Therefore, the photo-wind mill is maintained at its initial position adjacent to the first stopper 281.

The first stop 281 is a post that prevents counterclockwise movement of the phototool beyond the initial position shown in fig. 2A so that the reflected portion of the light beam is not erroneously partially or fully directed to the right side of the output port 225, for example, in response to external vibrations. For example, it may be formed of a material capable of buffering an impact force to reduce abrasion of the stopper 281 and a portion of the photo-windmill contacting the stopper 281.

Fig. 2B shows a relatively high power source beam entering the housing 240 through the input port 221, as shown by the relatively narrow transverse cross-hatching. (both uses of "opposite" in the previous sentence represent comparisons with fig. 2A.) in this case, the radiation flux of the light beam striking the first blade 211 is high enough to create a sufficient temperature gradient between the illuminated and shaded sides of the first blade 211, the windmill effect causing the windmill to rotate clockwise about the axis 214, away from the first stop 281 and towards the second stop 282. Thus, the input port 221 acts as a control port for the rotation of the windmill and the light beams shown entering and traveling within the housing 240 are all the source, transmission and control light beams as described above.

A biasing element (not shown) in the form of a spring attaching the windmill to the housing 240 is provided to slightly bias the windmill towards the position shown in figure 2A. (the resilient member may optionally be the shaft 214.) this reduces the risk of the phototool rotating clockwise (e.g. in response to external vibrations) under any influence other than the phototool effect. (the biasing element also increases the threshold radiant flux required to start rotation of the windmills relative to the windmills whose accelerations are limited only by their own inertia.) this properly calibrated biasing element may also be used to control the response of the limiter 200. This is because the rotation of the photo-windmill will stop at the point where the photo-windmill effect forces are balanced by the biasing force. For example, if the phototool is connected to the housing 240 via an elastic member, the angle at which the phototool rotates from the position shown in fig. 2A will be approximately proportional to the input beam power. Such biasing element may be adjustable; for example, the tension of the elastic member may be adjusted by winding or unwinding the elastic member from a reel. Suitable biasing elements may take other forms than resilient members, such as springs or magnetic devices.

With the windmill in the position shown in fig. 2B, a portion of the light beam is still substantially reflected toward the output port 225, but at an angle such that only some reflected portion of the light beam is incident on the output port 225, with the remainder blocked by the beam blocker 290. Thus, in fig. 2B, the radiant flux of the output beam exiting the housing 240 via the output port 225 is lower than the radiant flux of the source (/ control/transmission) beam entering the housing 240 via the input (/ control) port 221. It can be seen that the higher the radiant flux of a beam entering via the input port 221, the more the windmill will turn and therefore the smaller the area that the output port 225 will be illuminated. Thus, the mirror provided by the reflective surface of the first vane 211 together with the beam blocker 290 form an optical barrier arrangement arranged to prevent a portion of the beam from impinging on the output port 225, the size of the portion being dependent on the angle of rotation of the photo-mill.

A second limiting member 282, similar to the first limiting member 281, is provided by another post to set the maximum rotation angle of the windmill from the initial position shown in fig. 2A. This prevents the second blade 212 from rotating to the point of striking the housing 240. The positioning of the second limit stop 282 may be selected to have one of two effects on the response of the limiter 200 to high power inputs. If the second limiting member 282 is positioned far enough around the rotational path of the windmills that the windmills can rotate far enough that the light beam misses the output port 225 entirely, the limiter 200 will act to limit the output power to a certain input power and then effectively act as a fuse, causing the output power to drop to zero for all higher input powers. Alternatively, the second stop 282 may be positioned to limit the rotation of the phototool more so that if there is an input signal, the output radiant flux can be prevented from falling to zero at any time. That is, the second stop 282 may be positioned such that the clockwise rotation of the windmill stops just at the point where the reflected portion of the light beam completely misses the output port. (however, this would allow the output power to increase to a level above the limiter's intended limit, so if there is a risk of damage to network equipment due to a sudden increase in power above the limiter level, it may be advisable to use a spare optical fuse in conjunction with this type of limiter.)

The beam blocker 290 may be omitted from the limiter 200 and with the windmill in the position shown in figure 2B, a portion of the beam may still miss the output port 225 but instead strike the housing 240 near the output port 225. However, by using a dedicated beam blocker 290, the response of the limiter 200 can be controlled to be flatter than if the housing 240 surrounding the output port 225 were relied upon as part of a baffle arrangement.

Fig. 3A and 3B show schematic plan views of an example optical fuse 300 of similar design to the optical limiter 200 of fig. 2A and 2B. To avoid cluttering the two figures, reference numerals unrelated to the detailed description of each figure are omitted in this figure, although all components involved are present in the limiter 300 as shown in each of the two figures. The optical fuse 300 includes an optical windmill having a first blade 311 and a second blade 312, the first blade 311 and the second blade 312 configured to rotate about an axis 314. The optical windmill is enclosed in a housing 340 having an input port 321 and an output port 325. The rotation of the optical wind turbine is limited by the first limit member 381 and the second limit member 382. All of these components function in the same manner as the corresponding components of the limiter 200 of fig. 2A and 2B.

However, in contrast to the limiter 200, the fuse 300 does not include an elastic member existing in the limiter 200. In addition, the second blade 312 of the fuse 300 is magnetic (e.g., due to being made of iron or being coated with a layer of iron), and the fuse 300 also includes a biasing element in the form of a magnet 385. (in the design shown, the magnet 385 is outside the housing 340 for ease of adjustment, as will be described below, but it could also be within the housing 340.) the magnetic attraction between the magnet 385 and the second blade 312 keeps the windmill in abutment with the first stop 381 in the position shown in FIG. 3A as long as the radiant flux of the beam of light input through the input port 321 remains below the threshold.

If the radiant flux of the input beam reaches or exceeds this threshold, the phototool effect overcomes the magnetic attraction and the phototool (unconstrained by any elastic member) suddenly swings to the position shown in FIG. 3B, abutting the second stop 382, where the reflected portion of the beam misses the output port 325 completely, causing the output power to drop sharply to zero.

The optical paths in fig. 3A and 3B are represented by thin dashed and dotted lines (instead of the shaded areas in fig. 2A and 2B), respectively, because the sharp response of the fuse at the threshold input power means that the width of the beam is irrelevant. The entire width of the beam either passes through the output port 325 as shown in fig. 3A or misses the output port 325 altogether as shown in fig. 3B. (of course, when the output port 325 is illuminated by only a portion of the beam, there is a gap due to the windmill swinging between the positions shown in FIGS. 3A and 3B, but the gap is very brief.)

If and when the source beam is switched off, or its radiant flux drops below a threshold value, the magnet 385 causes the windmill to rotate back to the position shown in fig. 3A quickly counter-clockwise. Thus, the fuse 300 is reusable and self-resetting.

The magnet 385 has an outer member such that its position can be adjusted by screwing it closer or further from the second blade 312 within the internally threaded nut 386. In this way, the threshold power for opening the fuse may be adjusted. If the nut is long enough, the magnet 385 may even be retracted far enough from the second blade 312 that the fuse 300 responds in much the same way as the limiter 200. (in this case, a beam stop similar to beam stop 290 of limiter 200 may be added to flatten the response, as discussed above with respect to FIG. 2B.) thus, apparatus 300 may be multifunctional in nature; an adjustable, reusable, self-resetting optical fuse/limiter is provided.

Fig. 3C shows three-dimensionally the relative positions of the optical windmill (comprising the first blade 311 and the second blade 312 configured to rotate about the axis 314), the input port 321 and the output port 325, the first retaining member 381 and the second retaining member 382, and the magnet 385 of the fuse 300.

Figure 3D is a plan view of the housing 340 and the nut 386 of the fuse 300. The thickness of these parts is indicated by the dashed lines on their inner walls. Also shown in phantom are two

holes

341 and 345 in the housing 340 configured to facilitate coupling an input fiber (not shown) to the input port 321 and an output fiber (not shown) to the output port 325, respectively.

Holes

341 and 345 have a stepped profile with a relatively narrow inner portion (e.g., 3mm in diameter) and a relatively wide outer portion (e.g., 4.6mm in diameter). The input port 321 and the output port 325 are sealed within the interior portion such that the housing is airtight, thereby allowing the air within the housing to be maintained at an optimal low pressure for operation of the windmill. The outer portion is configured to receive an optical fiber (not shown) in a tight interference fit.

The housing 240 of the limiter 200 may be identical to the housing 340 of the fuse 300. The housing 240 of the limiter 200 and the housing 340 of the fuse 300 may be, for example, about 11mm high, 7mm wide and 27mm long, with the walls about 2mm thick. They may be made of, for example, plastic, metal, or another impermeable solid.

In both the limiter 200 and the fuse 300, a further port may be provided in the housing 240, 340 (not shown in any of fig. 2A to 3D) for a vacuum device to be attached, so that the air pressure within the housing may be maintained at an optimal level for operation of the photo-windmill, for example between 300mTor and 600 mTor.

The angle between the input and

output ports

221, 321 and 225, 325 in the limiter 200 and the fuse 300 may be, for example, an obtuse angle, for example, about 120 °.

The optical windmills of limiter 200 and fuse 300 may, for example, have blades that are about 1mm thick and 10mm long from axis to tip.

The entire limiter/

fuse assembly

200, 300 may have a mass of about 15g, for example.

The beam stop 290 of the limiter 200 may be, for example, about 3mm wide.

Additional exemplary limiters and fuses will now be described with reference to fig. 4A-9B. In these figures, conventions similar to those employed in fig. 2A-3B are used. Namely: (i) To avoid cluttering the drawings, not all reference numerals have been repeated among the several views of a particular device; (ii) Relatively wide and narrow cross-hatching is used in the depiction of the limiter to indicate the relatively low and high power beams; and (iii) thin dashed lines are used to represent relatively low power optical beams in the description of the fuse, and thin dashed lines are used to represent relatively high power optical beams.

Fig. 4A and 4B show schematic plan views of an example optical limiter 400 including an optical windmill in a first position and a second position, respectively. The limiter 400 functions in much the same way as the limiter 200 of fig. 2A and 2B. It comprises a windmill having a first blade 411 and a second blade 412 configured to rotate about an axis 414. Due to the connection to the housing 440 by the elastic member (not shown), the photo-windmill is biased to the position shown in fig. 4A. The optical windmill is enclosed in a housing 440 having an input port 421 and an output port 425. The rotation of the optical windmill is limited by the first and

second stoppers

481 and 482. A beam stop 490 is also provided. All of these components function in the same manner as the corresponding components of the limiter 200 of fig. 2A and 2B, the only difference being the geometry of their arrangement. Specifically, the input port 421 is perpendicular to the output port 425 in the limiter 400, in contrast to the input port 421 making an obtuse angle with the output port 425 in the limiter 200.

Fig. 5A and 5B show schematic plan views of an example optical fuse 500 designed similarly to the optical limiter 400 of fig. 4A and 4B. The optical fuse 500 includes an optical windmill having a first blade 511 and a second blade 512, the first blade 511 and the second blade 512 being configured to rotate about an axis 514. The optical windmill is enclosed in a housing 540 having an input port 521 and an output port 525. The rotation of the optical wind turbine is limited by the first stopper 581 and the second stopper 582. All of these components are identical to the corresponding components of the limiter 400 of fig. 4A and 4B.

However, in contrast to the limiter 400, there is no elastic element in the fuse 500, the second blade 512 is magnetic, and the fuse 500 further includes a biasing element in the form of a magnet 585. The magnet 585 functions in a similar manner to the magnet 385 of fig. 3A-3C, except that it is located within the housing 540 and is not adjustable. Thus, fuse 500 has a fixed threshold input power value that will cause the fuse to open and not act as a limiter. However, similar to fuse 300, fuse 500 is reusable and self-resetting.

Fig. 6A and 6B show schematic plan views of another example optical limiter 600 including an optical windmill in a first position and a second position, respectively. The limiter 600 functions in a manner similar to the limiter 400 of fig. 4A and 4B. It comprises an optical windmill enclosed in a housing 640 having an input port 621 and an output port 625. The phototool is attached to the housing 640 by a spring (not shown) that biases the phototool to the position shown in fig. 6A. A beam stop 690 is provided which functions in the same manner as the beam stop 490 of limiter 400.

However, the limiter 600 differs from the limiter 400 of fig. 4A and 4B in that the control of the rotation and direction of light from the input port 621 to the output port 625 is provided separately, rather than by the first blade of a windmill as in the limiter 400. As shown in fig. 6A and 6B, the source beam arriving from the left-hand side encounters a beam splitter 661, which beam splitter 661 splits the source beam into a control beam and a transmission beam.

The control beam is directed to a mirror 671 which directs the control beam through a control port 631 and onto the first blade 611 of the windmill. The first blade 611 absorbs some or all of the energy of the control beam. In the case where the source beam is relatively strong, as shown in fig. 6B, the control beam is strong enough to cause the windmills to rotate clockwise due to the windmilling effect caused by this absorption.

The transmitted beam continues through a beam splitter 661 and an input port 621 to be incident concentratedly on a light guide 613 such as a mirror. The light guide 613 is part of the rigid assembly of the windmill, is centered on the shaft 614 and is configured to rotate with the first blade 611 and the second blade 612 about the shaft 614. The transmitted light beam is reflected by the light guide at an angle that depends on the rotational position of the photo-windmill. When the windmill is in the position shown in fig. 6A, the transmission beam is incident on the output port 625 in a concentrated manner, so that the power of the output beam is maximized. With the windmill in the position shown in fig. 6B, a portion of the transmitted beam is blocked by beam blocker 690, and thus the power of the output beam is reduced. Thus, the light guide 613 and the beam stop 690 together form an optical barrier arrangement arranged to prevent a portion of the transmitted light beam, the size of which depends on the angle of rotation of the windmill, from impinging on the output port 625.

Any counterclockwise rotation of the photo-windmill caused by an effect other than the photo-windmill effect (e.g., external vibration) is constrained by the first limit member 681. The clockwise rotation of the photo-wind turbine is restricted by a pair of second stoppers 682. One of the pair of second stoppers 682 may be omitted, although the inclusion of two stoppers balances the forces on the two sides when the windmill abuts both stoppers, thereby reducing the risk of bending or breaking thereof. As described above with respect to fig. 2B, the positioning of the second limiter 682 determines the response of the amplitude limiter 600 to the high power source beam.

Fig. 7A and 7B show schematic plan views of an example optical fuse 700 of similar design to the optical limiter 600 of fig. 6A and 6B. The optical fuse 700 includes a photo-wind wheel having a first blade 711, a second blade 712, and a light guide 713 rigidly connected therebetween. All of the first blade 711, the second blade 712, and the light guide 713 are configured to rotate together about the axis 714. The optical windmill is enclosed in a housing 740 having an input port 721, an output port 725, and a control port 731. The phototool is attached to the housing 740 by a spring (not shown) that biases the phototool to the position shown in fig. 7A. Via reflection by the mirror 771, the beam splitter 761 is configured to split the source beam into a transmission beam incident on the input port 721 and a control beam incident on the control port 731. The rotation of the optical wind turbine is restricted by the first limit member 781 and the pair of second limit members 782. All of these components function in the same manner as the corresponding components of the limiter 600 of fig. 6A and 6B.

However, in contrast to the limiter 600, there is no elastic element in the fuse 700, the second blade 712 is magnetic, and the fuse 700 further includes a biasing element in the form of a magnet 785. The magnet 785 functions in the same manner as the magnet 585 of the fuse 500.

Fig. 8A and 8B show schematic plan views of another example optical limiter 800 including an optical windmill in a first position and a second position, respectively. The limiter 800 functions in a manner similar to the limiter 600 of fig. 6A and 6B. It comprises an optical windmill enclosed in a housing 840, the housing 840 having an input port 821, an output port 825, and a control port 831. The phototool is attached to the housing 840 by a spring (not shown) that biases the phototool toward the position shown in fig. 8A. The optical splitter 861 is configured to split the source beam into a transmission beam incident on the input port 821 and a control beam incident on the control port 831, as in the limiter 600 of fig. 6A and 6B, by reflection from the mirror 871. The clockwise rotation of the windmill is constrained by a pair of stoppers 882 corresponding to a pair of second stoppers 682 of the limiter 600.

However, limiter 800 differs from limiter 600 of fig. 6A and 6B in that the windmills of limiter 800 do not include optical directors. Limiter 800 has an optical barrier arrangement comprising two

beam blockers

891 and 892 bounding an aperture. (alternatively, the optical barrier means may be provided by a single annular beam blocker which appears the same when passing through the cross-section of the horizontal plane of the aperture.)

beam blockers

891 and 892 are rigidly connected between first blade 811 and second blade 812 and are configured to rotate with them about an axis not shown (as it is attached to the shown component of the photo-mill above and/or below the horizontal plane of the aperture). The output port 825 is parallel and coaxial with the input port 821. When the windmill is in the position shown in figure 8A, the aperture between the

beam blockers

891 and 892 is intermediate and coaxial with the input port 821 and the output port 825 so that the power of the output beam is maximized. As shown in fig. 8B, when the optical windmill is rotated from the position shown in fig. 8A to the position shown in fig. 8B, the increased area of the transmitted light beam is blocked by the

beam blockers

891 and 892, so that the power of the output light beam is reduced.

Although a limit member may be provided, it is not shown that the limit member holds the windmill in the initial position shown in fig. 8A against an effect other than the windmill effect, such as external vibration. For example, one or both of first blade 811 and second blade 812 and/or the spokes on which they are mounted may be configured to weakly attract to one or more magnets in corresponding locations on the base and/or top of housing 840. Alternatively, the resilient member may be sufficient to perform this function.

Fig. 9A and 9B show schematic plan views of an example optical fuse 900 designed similarly to the optical limiter 800 of fig. 8A and 8B. Optical fuse 900 includes an optical windmill having a first blade 911, a second blade 912, and two

beam blockers

991 and 992, which

beam blockers

991 and 992 define apertures, rigidly connected between first blade 911 and second blade 912. All of first blade 911 and second blade 912 and first beam blocker 991 and second beam blocker 992 are configured to rotate together about an axis (not shown). The optical windmill is enclosed in a housing 940 having an input port 921, an output port 925 and a control port 931. The beam splitter 961 is configured to split the source beam into a transmission beam incident on the input port 921 and a control beam incident on the control port 931 via reflection by the mirror 971. All of these components are identical to the corresponding components of limiter 800 of fig. 8A and 8B.

However, in contrast to limiter 800, there is no resilient element in fuse 900, second blade 912 is magnetic, and fuse 900 also includes a biasing element in the form of a magnet 985. The magnet 985 functions in the same manner as the magnet 785 of the fuse 700.

The fuse 900 further includes a first limiting member 981 and a second limiting member 982 to respectively restrict counterclockwise rotation and clockwise rotation of the windmills in a manner similar to the first limiting member 581 and the second limiting member 582 of the fuse 500 of fig. 5A and 5B.

In all of the example limiters and fuses described above, at least one region of at least one face of at least one blade of each optical windmill is configured to have light incident thereon and absorb energy from the light such that the light warms the at least one region relative to an opposing region of the opposite face of the blade. The absorption area may for example be coated with graphite, black aluminium foil, anodised aluminium or Litho-Black ^TM . If the opposing regions are thermally insulated from each other and the absorbing regions are illuminated to a greater extent than the opposing regions, there need not be any asymmetry in their optical absorption. (optical absorption is defined as the ratio of absorbed radiation power to incident radiation power.) the illuminating light as a laser beam is sufficiently narrow relative to the size of the blade that targeting only one side of the blade enhances the effect. However, by providing an absorption region having a higher optical absorptance than the opposite region, the phototool effect can be enhanced. For example, the opposite area may be covered with a reflective metal, such as silver or a dielectric material. Alternatively or additionally, the absorption region may be shaped such that it receives a greater amount of radiant flux within the allowed rotational range of the windmill than the opposite region, using the principles described above with respect to fig. 1C.

The optical windmill of all of the above exemplary fuses and limiters comprises two blades; a first blade configured to be struck by the control beam and a second blade that acts as a counterweight. The second blade may be omitted and the phototool will still rotate in response to the control beam. Alternatively, the windmill may be provided with more than two blades.

Some example limiters and fuses described above include one or more light directors, such as mirrors. Such a mirror may be provided by an at least partially reflective surface. If the mirror needs to be able to absorb some light, such as in the exemplary switches and

limiters

200, 300, 400, and 500 described above with respect to fig. 2A-5B, its surface may be made partially reflective, such as by laminating a thin dielectric on a light absorbing surface (e.g., a surface coated with a light absorbing material as described above). Other optical components, such as prisms, may also be used as light guides.

In all of the above exemplary limiters and fuses, at least one component of the optical barrier arrangement is arranged to rotate with the windmill. However, other arrangements may be envisaged in which the movement of the windmill causes redirection or blocking of the transmitted light beam in some other way. For example, a cam arrangement may be used to convert the rotational motion of a photo-windmill into linear motion of a light guide.

In all of the above described exemplary limiters and fuses, the movement of the phototools is constrained by a stop in the form of a bumper/shock absorber/support element provided for one or more of the phototool blades (and/or the spokes carrying them), but abuts upwardly. Alternatively, a single stop may be provided for a plurality of blades, for example such that in the two-blade example the windmill rotates almost a full turn between its two positions.

Other forms of limiters may also be used; elements that prevent or impede rotation in one direction past a certain position while allowing (some) counter-rotation away from that position are suitable. For example, other types of mechanical stops, such as clips (catch), are also contemplated, in addition to the magnetic stops described above.

Optical transmission (input/output) ports and control ports used in limiters and fuses according to the present disclosure may be used to couple light from an optical fiber to an optical fiber. They may optionally include lenses to focus or defocus the light appropriately.

Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the embodiments disclosed herein. The specification and embodiments are to be considered exemplary only.

Furthermore, where the application has listed the steps of a method or process in a particular order, it may be possible, or even advantageous in certain circumstances, to change the order in which some steps are performed, and unless such order specificity is explicitly stated in the claims, specific steps of the method or process claims set forth herein are not to be construed as order-specific. That is, the operations/steps may be performed in any order, unless otherwise specified, and embodiments may include more or less operations/steps than those disclosed herein. It is further contemplated that executing or performing a particular operation/step before, contemporaneously with, or after another operation is in accordance with the described implementations.

The claims (modification of treaty clause 19)

1. An optical limiter for limiting radiant flux of a source beam, said limiter comprising:

a light control port illuminated by a light control beam originating from the source light beam;

an optical input port illuminated by a light transmitting beam originating from the source light beam;

an optical output port illuminated by the optical transmission beam; and

a thermally driven photo-windmill;

wherein the optical windmill is arranged relative to the optical input port, the optical control port, and the optical output port such that:

the optical windmill is driven to rotate by illumination of the optical control port by the optical control beam only if the optical control beam has a radiant flux equal to or exceeding a predetermined radiant flux threshold; and

rotation of the optical windmill causes the area of the optical output port illuminated by the optical transmission beam to change.

2. The optical limiter of claim 1 wherein the optical limiter is configured to allow the windmill to rotate an angle large enough to zero the area of the optical output port illuminated by the optically transmitted beam such that the limiter acts as a reusable optical fuse.

3. The optical limiter of claim 1 or 2, wherein the optical limiter is configured to limit a radiant flux of a source beam originating from a laser.

4. The optical limiter of any one of claims 1 to 3, wherein:

the side of a blade of the photo-mill that is arranged to be illuminated in use by the light control beam has a higher light absorption rate than the opposite side of the blade; and/or

The side of the blade of the photo-windmill that is arranged to be illuminated by the light-control beam in use and the opposite side of the blade are shaped such that, in use, the side of the blade that is illuminated by the light-control beam receives a greater amount of radiant energy from the light-control beam than the opposite side within the permitted range of rotation of the photo-windmill.

5. The optical limiter of any one of the preceding claims, wherein the optical limiter further comprises an optical baffle arrangement arranged to prevent a portion of the optically transmitted light beam from impinging the optical output port, the size of the portion being dependent on the angle of windmilling.

6. The optical limiter of claim 5 wherein said optical baffle arrangement comprises:

one or more light guides; and/or

One or more beam blockers.

7. The optical limiter of claim 5 or 6, wherein:

the optical input port and the optical output port are coaxial with each other;

the optical baffle device includes one or more beam blockers configured to rotate with the phototool; and

the one or more beam blockers are arranged to define an aperture through which the light-transmitting beam must pass to reach the light output port.

8. The optical limiter of any one of claims 1 to 6 wherein the optical input port is the optical control port, the source light beam itself serving in use as the optical transmission beam and the optical control beam.

9. The optical limiter of any one of claims 1 to 7 further comprising an optical splitter arranged to split the source light beam into the light transmission beam and the light control beam.

10. An optical limiter according to any one of the preceding claims, further comprising a biasing element;

wherein the phototool is coupled to the biasing element such that the phototool is biased toward a first rotational position in which an area of the light output port illuminated by the light transmission beam is maximized relative to any other rotational position of the phototool.

11. The optical limiter of claim 10, wherein the biasing element is configured to be adjustable such that a biasing force exerted by the biasing element on the windmill can be varied.

12. The optical limiter of claim 10 or 11 as dependent directly or indirectly on claim 2, wherein the biasing element is configured to:

preventing rotation of the phototool from the first rotational position when the radiant flux of the source beam is below the predetermined radiant flux threshold; and

allowing the windmill to rotate from the first rotational position by a sufficient angle when the radiant flux of the source beam equals or exceeds the predetermined radiant flux threshold such that the area of the light output port illuminated by the light transmission beam is zero.

13. An optical limiter according to any one of the preceding claims further including one or more limiters, each limiter being arranged to prevent rotation of the windmill beyond a particular rotational position.

14. The optical limiter of any one of the preceding claims, further comprising a housing surrounding the phototool, the housing including an aperture configured to partially expel fluid around the phototool.

15. A method of limiting radiant flux of a source beam, the method comprising:

illuminating the optical input port of the optical limiter according to any one of the preceding claims with a light transmission beam originating from the source beam; and

illuminating the light control port of the optical limiter according to any one of the preceding claims with a light control beam originating from the source beam.

Claims

an optical control port illuminated by an optical control beam originating from the source beam;

an optical output port illuminated by the transmission beam; and

a thermally driven photo-windmill;

2. The optical limiter of claim 1 wherein the optical limiter is configured to allow the windmill to rotate an angle large enough to zero the area of the output port illuminated by the transmitted beam of light such that the limiter acts as a reusable optical fuse.

3. The optical limiter of claim 1 or 2, wherein the source beam is laser-derived.

4. The optical limiter of any one of claims 1 to 3, wherein:

a side surface of a blade of the photo-wind turbine that is disposed to be irradiated with the control beam has a higher light absorption rate than an opposite side surface of the blade; and/or

The side of the blade of the photo-mill that is arranged to be illuminated by the control beam and the opposite side of the blade are shaped such that the side of the blade that is illuminated by the control beam receives a greater amount of radiant energy from the control beam than the opposite side, within the allowed rotation range of the photo-mill.

5. An optical limiter according to any one of the preceding claims further comprising an optical baffle arrangement arranged to prevent a portion of the transmitted beam from illuminating the output port, the size of the portion being dependent on the angle of rotation of the phototool.

6. The optical limiter of claim 5 wherein said baffle arrangement comprises:

one or more light guides; and/or

One or more beam blockers.

7. The optical limiter of claim 5 or 6, wherein:

the input port and the output port are coaxial with each other;

the barrier device comprises one or more beam blockers arranged to rotate with the photo-windmill; and

the one or more beam blockers are arranged to define an aperture through which the transmitted beam must pass to reach the output port.

8. The optical limiter of any one of claims 1 to 6 wherein the input port is the control port, the source beam itself serving as the transmission beam and the control beam.

9. The optical limiter of any one of claims 1 to 7 further comprising an optical splitter arranged to split the source beam into the transmission beam and the control beam.

11. The optical limiter of claim 10, wherein the biasing element is configured to be adjustable such that a biasing force exerted by the biasing element on the phototool can be varied.

preventing the light windmill from rotating away from the first rotational position when the radiant flux of the source beam is below the predetermined threshold; and

when the radiant flux of the source beam equals or exceeds the predetermined threshold, the photo-windmill is allowed to rotate from the first rotational position by a sufficiently large angle such that the area of the output port illuminated by the transmission beam is zero.

14. The optical limiter of any one of the preceding claims, further comprising a housing surrounding the windmill, the housing comprising an aperture configured to partially drain fluid around the windmill.

15. A method of limiting radiant flux of a source beam, the method comprising:

illuminating the input port of the optical limiter according to any one of the preceding claims with an optical transmission beam originating from the source beam; and

illuminating the control port of an optical limiter according to any one of the preceding claims with an optical control beam originating from the source beam.